Circuit board with an attached die and intermediate interposer

ABSTRACT

A die having a base formed of a first material is connected to a board having a base formed of a second material. An interposer having a coefficient of thermal expansion intermediate coefficients of thermal expansion of the first and second materials is positioned between the die and the board.

BACKGROUND OF THE INVENTION

This application relates to a method of attaching a die, such as aswitch, made of a first base material to a circuit board made of asecond base material. An interposer having a coefficient of thermalexpansion intermediate the first and second materials is used to reducethermal stresses.

Circuit boards are utilized in various control applications. As oneexample, modern aircraft have many printed circuit boards providingvarious control functions. One known circuit board has a die attached tothe circuit board, to provide a control function. As an example, an FET(field effect transistor) may provide On/Off switch and circuit breakerfunctions to a circuit on the circuit board.

In many applications the circuit board has a base, such as a polymerbase, while the attached die may have a silicon base. When the two areattached together, there is a good deal of thermal stress between thetwo due to their differing coefficients of thermal expansion. As anexample, if the two are soldered together, the solder absorbs thedifferential expansion and may crack. It has sometimes been challengingto provide such an assembly that is able to withstand expected thermalexpansion over a wide temperature range.

SUMMARY OF THE INVENTION

In a disclosed embodiment, an interposer material is positioned betweenthe die and the circuit board, and is formed of a material having acoefficient of thermal expansion intermediate the coefficient of thermalexpansion of the die and the material of the circuit board. Theinterposer also forms a conductive connection between the die and thecircuit boards. In disclosed embodiments, various types of interposersmay be utilized.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view through a circuit board and connecteddie.

FIG. 2 shows a first embodiment interposer.

FIG. 3 shows a second embodiment interposer.

FIG. 4 shows a third embodiment interposer.

FIG. 5 shows a fourth embodiment interposer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A printed circuit board assembly 20 is illustrated in FIG. 1 having aprinted board 22 attached to a die 24. The die may be a field effecttransistor, or some other type of solid state switch, and may provide acircuit breaker function to circuits on the printed board 22. Otherswitches and functions may also be used. The die 24 has a base of afirst material along with circuit traces top and bottom, as known. Theprinted board 22 has a base of a second material, along with circuittraces. The particular disclosed embodiment has a silicon base die and aglass epoxy base printed board, but other materials may be utilized forthe two members. The circuit traces are shown schematically at C and maybe as known, to provide various functions.

The base materials for the die 24 and the printed board 22 are distinct,and have different coefficients of thermal expansion. As such, thermalstresses may exist at an interface between the two materials.

With the present application, a solder or other conductive material,such as a conductive adhesive is formed in one layer adjacent die 24,and another layer 28 adjacent the printed board 22. An intermediatematerial provides an interposer 30. Openings 32 within the materialreceive a conductive material to provide electrical connection betweenlayers 26 and 28. The interposer 30 is selected to be of a materialhaving a coefficient of thermal expansion intermediate that of the basematerials of die 24 and the printed board 22. Thus, the thermal stressesbetween the die and printed board are reduced due to the interposer.

FIG. 2 shows a first embodiment interposer 130, wherein the layers 26and 28 are connected by plated through holes 32 formed within theinterposer material. The interposer material includes material 34, suchthat the thermal coefficient of expansion is intermediate that of thedie 24 and the printed wiring board 22.

FIG. 3 shows another interposer embodiment 230 wherein a metal screen232 is utilized. Solder or conductive adhesive flows through theopenings 234 in the screen 232.

FIG. 4 shows another embodiment 330, wherein the interposer includes amaterial 332 formed with a plurality of holes 334. The holes are shownas round, but can be a plurality of shapes. Solder or conductivematerial flows through these openings to provide the electricalconnections.

FIG. 5 shows another embodiment 50, wherein the printed board 52 isconnected to the die 54 by an interposer 60. As shown, a conductivelayer 58 is formed on the printed board 52, while a conductive layer 56is provided adjacent the die 54. The interposer includes a back 62 incontact with the layer 56, and legs 60 which connect to the conductiveportions 58 for the printed wiring board 52. The interposer itself isformed of a conductive metal to make the electrical connection. A space66 is provided between the die 54 and the board 52 with this embodiment.This space can maximize the compliance between the printed wiring board52 and the die 54. Other shapes can be utilized for the interposer.

The interposer can be made from any material having the coefficient ofthermal expansion as mentioned above. One possible material would be apowdered metal injection molding compound. The process for formingmaterials from this compound could be optimized to obtain a specificcoefficient of expansion. The porosity of this type material will allowthe solder or other conductive type material to provide a superiorelectrical connection.

Another general type of material may be polyimide of bismuth tellurideprinted wiring board material. Again, an interposer would be plated onthe top and bottom with plated through holes to provide an electricalconnection between the top and bottom, such as shown for example in FIG.2 above.

With all of the embodiments, it should be understood that the locationof the conductive connection between the die and printed board, andthrough the interposer, is selected to provide appropriate electricalconnections between the circuitry on the die and the printed board. Theillustrations are a simplification of the actual likely connections.

Although embodiments of this invention have been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

1. A printed circuit board comprising: a board having a substrate formedof a first material having a first coefficient of thermal expansion, andhaving circuit traces; a die secured to said board, said die having abase formed of a second material having a second coefficient of thermalexpansion, different than said first coefficient of thermal expansionand circuit traces, and said die circuit traces being electricallyconnected to said board circuit traces; an interposer materialpositioned between said die and said board, said interposer having anelectrical connection connecting said die circuit traces to said boardcircuit traces, said interposer being formed of a third material havinga coefficient of thermal expansion intermediate said first and secondcoefficients of thermal expansion; said die being a switch; and saidinterposer material being selected to be an electrically conductivematerial such that there is an electrical connection between said dieand said board across said interposer material.
 2. The printed circuitboard as set forth in claim 1, wherein said switch is a field effecttransistor.
 3. The printed circuit board as set forth in claim 1,wherein a first layer of conductive material is positioned between saidinterposer and said die, and a second layer of conductive material ispositioned between said interposer and said board, with said connectionextending through said interposer for connecting said first and secondlayers.
 4. The printed circuit board as set forth in claim 1, whereinsaid first material is a glass filled epoxy.
 5. The printed circuitboard as set forth in claim 4, wherein said second material is silicon.6. The printed circuit board as set forth in claim 1, wherein saidinterposer is formed of a powdered metal injection molding compound. 7.The printed circuit board as set forth in claim 1, wherein saidinterposer includes a base portion and legs with said legs positioningsaid die away from said board by a space.
 8. The printed circuit boardas set forth in claim 7, wherein said legs also provide an electricalconnection between said die and said board.
 9. The printed circuit boardas set forth in claim 1, wherein said interposer is formed of apolyimide of bismuth telluride.
 10. A printed circuit board comprising:a board having a substrate formed of a glass filled epoxy having a firstcoefficient of thermal expansion, and having circuit traces; a switchsecured to said board, and having a base formed of a silicon having asecond coefficient of thermal expansion, different than said firstcoefficient of thermal expansion, and said switch being electricallyconnected to said board; an interposer material positioned between saidswitch and said board, said interposer having an electrical connectionconnecting said switch to said board, said interposer having acoefficient of thermal expansion intermediate said first and secondcoefficients of thermal expansion; and said interposer material beingselected to be an electrically conductive material such that there is anelectrical connection between said die and said board across saidinterposer material.
 11. The printed circuit board as set forth in claim10, wherein said switch is a field effect transistor.
 12. The printedcircuit board as set forth in claim 10, wherein a first layer ofconductive material is positioned between said interposer and saidswitch, and a second layer of conductive material is positioned betweensaid interposer and said board, with said connection through saidinterposer connecting said first and second layers.
 13. The printedcircuit board as set forth in claim 10, wherein said interposer includesa base portion and legs with said legs positioning said switch away fromsaid board by a space.
 14. The printed circuit board as set forth inclaim 13, wherein said legs also provide an electrical connectionbetween said switch and said board.
 15. The printed circuit board as setforth in claim 14, wherein said base portion is in contact with saidswitch via a first layer of conductive material, and wherein said legsare in contact with said board via a second layer of conductivematerial.